Competition of terrace and step-edge sputtering under oblique-incidence ion impact on a stepped Pt(1 1 1) surface

Using molecular-dynamics simulation, we study the sputtering of a Pt(1 1 1) surface under oblique and glancing incidence 5 keV Ar ions. For incidence angles larger than a critical angle ?_c, the projectile is reflected off the surface and the sputter yield is zero. We discuss the azimuth dependence of the critical angle ?_c with the help of the surface corrugation felt by the impinging ion. If a step exists on the surface, sputtering occurs also for glancing incidence ?>?_c. We demonstrate that for realistic step densities, the total sputtering of a stepped surface may be sizable even at glancing incidence.     

Grazing scattering of 1-2 MeV HeH ions from KCl(0 0 1): Effect of surface track potential

Three dimensional (3D) distributions (energy E, scattering angle θ and azimuth angle φ) of the fragment protons dissociated from HeH⁺ during grazing angle scattering from a KCl(0 0 1) are measured using a magnetic spectrometer in order to study the effect of the surface track potential. The distributions of the fragment protons scattered from a SnTe(0 0 1) are also measured as a reference. Although the observed distributions for KCl(0 0 1) and SnTe(0 0 1) are basically the same, there is small differences, especially in the scattering angle distribution. While the fragment protons are scattered at the specular angle from SnTe(0 0 1), the protons are scattered at slightly larger angles from KCl(0 0 1). The observed angular shift is more pronounced for the trailing protons than the leading protons. It is also found that the angular shift increases with decreasing ion energy. The observed angular shift can be qualitatively explained by the surface track potential induced by the partner He ions using a simple model of the surface track potential.     

Sputtering and surface state evolution of Bi under oblique incidence of 120 keV Ar ions

The sputtering and surface state evolution of Bi/Si targets under oblique incidence of 120 keV Ar⁺ ions have been investigated over the range of incidence angles 0° ? θ_i ? 60°. Increasing erosion of irradiated samples (whose surface thickness reduced by ∼3% at normal incidence up to ∼8% at θ = 60°) and their surface smoothing with reducing grain sizing were pointed out using Rutherford backscattering (RBS), atomic force (AFM) and X-ray diffraction (XRD) techniques. Measured sputtering yield data versus θ_i with fixed ion fluence to ∼1.5 × 10¹⁵ cm⁻² are well described by Yamamura et al. semi-empirical formula and Monte Carlo (MC) simulation using the SRIM-2008 computer code. The observed increase in sputter yield versus incidence angle is closely correlated to Bi surface topography and crystalline structure changes under ion irradiation.     

PL and XPS study of radiation damage created by various slow highly charged heavy ions on GaN epitaxial layers

Photoluminescence (PL) spectrum, in conjunction with X-ray photoelectron spectroscopy (XPS) is used to evaluate the surface damage of GaN layer by highly-charged Xe^q+ (18 ? q ? 30), Ar^q+ (6 ? q ? 16) and Pb^q+ (q = 25,35) ions. The intensity of PL emission of GaN layer, including near band-edge peak and yellow luminescence, decreases with increasing fluence and charge state of the incident ions. Finally the PL emission is completely quenched after irradiation to high fluences at high charge state. A new peak at 450 nm appeared in PL spectra of the specimens irradiated with Xe¹⁸⁺, Ar⁶⁺ and Ar¹¹⁺, indicating that radioactive recombination within donor-acceptor pairs (DAPs) during irradiation. After irradiation, XPS spectra show N deficient or Ga rich on GaN surface and XPS spectra of Ga3d core levels indicate spectral peak evidently shifts from a Ga-N to Ga-Ga and Ga-O bond. The relative content of Ga-N bond decreases and the content of Ga-Ga bond increases with the increase of ion fluence and ion charge state. The binding energy of Ga3d_5/2 electron corresponding to Ga-Ga bond of the irradiated GaN film is found to be smaller than that of metallic Gallium (Ga⁰), which can be attributed to irradiation damage.     

Angular dependence of electronic sputtering from HOPG

We have studied the angular distribution of 120 MeV Au ion beam induced sputtering yield for three cases: from crystalline highly oriented pyrolytic graphite (HOPG) for (A) normal and (B) 70° incidence and from (C) amorphous carbon sample for normal incidence. An anisotropic distribution of sputtering is observed for HOPG samples studied with a distribution Y = Acosⁿθ + Bexp[−(θ − μ)²σ²]. Though the over-cosine function dependence is observed for all the cases, the anomalous peak observed at 53° for normal incidence for HOPG sample is found to shift to 73° when the sample is tilted by 20°. No peak is observed in the amorphous carbon sample which further confirms that the anisotropy observed is due to the crystal structure and formation of a pressure pulse. The high exponent of over-cosine distribution of sputtering yield (n = 3.2-3.8) signifies formation of intense pressure pulse induced jet like sputtering.     

Guiding and blocking of highly charged ions through a single glass capillary

Highly charged ions produced in an electron beam ion trap, I^q+, q = 10-50, were transmitted through a tapered glass capillary having diameter of at the end. We found that for a particular beam current, there exists an optimum tilting angle of the capillary in which a steady output of ions is observed, while for smaller angles, the ion counts first rise, then gradually decay on a time scale of minutes. In the case of steady transmission, the charge state distribution is found to be slightly towards the lower side.     

Erosion of Ag surface by continuous irradiation with slow, large Ar clusters

Molecular dynamics computer simulations are employed to probe processes taking place during continuous irradiation of Ag(1 1 1) surface by keV Ar₈₇₂ projectiles. Surface modification, the total sputtering yield, and the angular distributions of ejected species are calculated at fluences ranging from 0 up to ∼6 × 10¹³ impacts/cm². It has been shown that two trends can be identified in the development of surface roughness. At the beginning surface roughness increases fast. This fast increase terminates around 1 × 10¹³ impacts/cm² and is followed by a slow increase that finally saturates. The effect of the surface roughness on the sputtering yield depends on the impact angle. At normal incidence the sputtering yield is rather insensitive to the development of the surface topography. Modification of the surface morphology has, however, a significant influence on the total sputtering yield at large impact angles. Both the shape of the sputtering yield dependence on the impact angle and the angular spectra of ejected particles are sensitive to the surface roughness.     

Incidence-angle dependent Cs incorporation in Si during low-energy bombardment: A dynamic computer simulation study

The implantation of Cs atoms in silicon was investigated by dynamic computer simulations using the Monte-Carlo code T-DYN that takes into account the gradual change of the target composition due to the Cs irradiation. The incorporation of Cs atoms was studied for incidence angles ranging from 0° to 85° and for four impact energies (0.2, 0.5, 1 and 3 keV). The total implantation fluences were (1-2) × 10¹⁷ Cs/cm², well above the values required to reach a stationary state. The steady-state Cs surface concentrations exhibit a pronounced dependence on impact angle and energy. At normal incidence, they vary between ∼0.57 (at 0.2 keV) and ∼0.18 (3 keV), but decrease with increasing incidence angle. Under equilibrium, the partial sputtering yield of Si exhibits the typical dependence on incidence angle, first increasing up to a maximum value (at ∼70°-75°) and declining sharply for larger angles. For all irradiation conditions a strongly preferential sputtering of Cs as compared to Si atoms is found, increasing with decreasing irradiation energy (from 4.6 at 3 keV to 7.2 at 0.2 keV) and for nearer-normal incidence.     

On the emission of MoCs molecular ions from Cs irradiated molybdenum surface

The present paper deals with the emission of atomic and molecular ions from elemental molybdenum surface under Cs⁺ bombardment to explore the MCs⁺ formation mechanism with changing Cs surface coverage. Integrated count of MoCs⁺ shows a monotonic increase with increasing primary ion energy (1-5 keV). Change in MoCs⁺ intensity is attributed to the variation of surface work function ? and cesium surface concentration c_Cs due to varying impact energies. Variation of c_Cs has been obtained from the expression, c_Cs ∝ 1/(1 + Y) where Y is the elemental sputtering yield estimated from TRIM calculations. Systematic study of the energy distributions of all species emerging from Mo target has been done to measure the changes in surface work function. Changing slopes of the leading parts of Cs⁺ energy distributions suggest a substantial depletion in surface work function ? with decreasing primary ion energies. Δ? shows a linear dependence on c_Cs. The maximum reduction in surface work function Δ?_max = 0.69 eV corresponds to the highest value of c_Cs = 0.5. A phenomenological model, based on the linear dependence of ? on c_Cs, has been employed to explain the MoCs⁺ data.     

Influence on the structural characteristic of defect solid solution GdxZr1−xO2−x/2 with 0.18 ? x ? 0.62 under longer term annealing studied by Gd Mössbauer spectroscopy

Three kinds of defect solid solution Gd_xZr_1−xO_2−x/2 with 0.18 ? x ? 0.62, including the three single crystal samples with x = 0.21, 0.26 and 0.30, were investigated by ¹⁵⁵Gd Mössbauer spectroscopy at 12 K. Difference in the structural characteristic under longer term annealing were confirmed by comparing the ¹⁵⁵Gd Mössbauer parameters of the polycrystalline samples sintered one time and twice at 1773 K for 16 h in air, respectively. The results indicated that the polycrystalline samples sintered twice have relatively equilibrated structure by comparing with the three single crystal samples. After being sintered twice, basically the local structure around the Gd³⁺ ions does not change, but the degree of the displacements of the six 48f oxygen ions from positions of cubic symmetry becomes slightly smaller, and distribution of the Gd³⁺ ions in the system becomes more homogeneous.     

Electronic sputtering of Gd3Ga5O12 and Y3Fe5O12 garnets: Yield, stoichiometry and comparison to track formation

The results of present paper have shown that sputtering of yttrium iron garnet (Y₃Fe₅O₁₂) under swift heavy ions in the electronic energy loss regime is non-stoichiometric. Here we are presenting additional experimental results for gadolinium gallium garnet (Gd₃Ga₅O₁₂) as target. The irradiations were performed with different ions (⁵⁰Cr (589 MeV), ⁸⁶Kr (195 MeV) and ¹⁸¹Ta (400 MeV)) impinging perpendicularly to the surface. As earlier, the sputtering yield was determined by collecting the emitted gadolinium and gallium atoms on a thin aluminium foil, placed upstream above the target and analyzing the Al catcher by Rutherford backscattering. Also for Gd₃Ga₅O_12, the emission of Gd and Ga is non-stoichiometric. Sputtering appears above a critical electronic stopping power of S_th = 11.6 ± 1.5 keV/nm, which is larger than the threshold for track formation, in agreement with other amorphisable materials. In addition, the angular distribution of the sputtered species was measured for Y₃Fe₅O₁₂ and Gd₃Ga₅O₁₂ using 200 MeV Au ions impinging the surface at 20° relatively to the surface. For the two garnets the ratio of Y/Fe (and Gd/Ga) varies with the angle of emitted species and the stoichiometry seems to be preserved only for an emission perpendicular to the surface.     

Separation of potential and kinetic electron emission from Si and W induced by multiply charged neon and argon ions

The total secondary electron emission yields, γ_T, induced by impact of the fast ions Ne^q+ (q = 2-8) and Ar^q+ (q = 3-12) on Si and Ne^q+ (q = 2-8) on W targets have been measured. It was observed that for a given impact energy, γ_T increases with the charge of projectile ion. By plotting γ_T as a function of the total potential energy of the respective ion, true kinetic and potential electron yields have been obtained. Potential electron yield was proportional to the total potential energy of the projectile ion. However, decrease in potential electron yield with increasing kinetic energy of Ne^q+ impact on Si and W was observed. This decrease in potential electron yield with kinetic energy of the ion was more pronounced for the projectile ions having higher charge states. Moreover, kinetic electron yield to energy-loss ratio for various ion-target combinations was calculated and results were in good agreement with semi-empirical model for kinetic electron emission.     

Growth of silicon nitride films by bombarding amorphous silicon with N ions: MD simulation

In this study, the molecular dynamics simulation method was employed to investigate the growth of silicon nitride films by using N⁺ ions, with energies of 50, 100, 150 and 200 eV, to bombard an amorphous silicon surface at 300 K. After an initial period of N⁺ bombardment, saturation of the number of N atoms deposited on the surface is observed, which is in agreement with experiments. During subsequent steady state deposition, a balance between uptake of N by the surface and sputtering of previously deposited N is established. The Si(N_x) (x = 1-4) and N(Si_y) (y = 1-3) bond configurations in the grown films are analyzed.     

Light ion irradiation effects on stuffed Lu2(Ti2−xLux)O7−x/2 (x = 0, 0.4 and 0.67) structures

We have recently synthesized “stuffed” (i.e., excess Lu) Lu₂(Ti_2−xLu_x)O_7−x/2 (x = 0, 0.4 and 0.67) compounds using conventional ceramic processing. X-ray diffraction measurements indicate that stuffing more Lu³⁺ cations into the oxide structure leads eventually to an order-to-disorder (O-D) transition, from an ordered pyrochlore to a disordered fluorite crystal structure. At the maximum deviation in stoichiometry (x = 0.67), the Lu³⁺ and Ti⁴⁺ ions become completely randomized on the cation sublattices, and the oxygen “vacancies” are randomized on the anion sublattice. Samples were irradiated with 400 keV Ne²⁺ ions to fluences ranging from 1 × 10¹⁵ to 1 × 10¹⁶ ions/cm² at cryogenic temperatures (∼77 K). Ion irradiation effects in these samples were examined by using grazing incident X-ray diffraction. The results show that the ion irradiation tolerance increases with disordering extent in the non-stoichiometric Lu₂(Ti_2−xLu_x)O_7−x/2.     

Photoconduction investigations in 75 MeV oxygen ion irradiated kapton-H polyimide

Photoconduction behaviour of 75 MeV oxygen ion irradiated (Fluences: 1.8 × 10¹¹, 1.8 × 10¹² and 1.8 × 10¹³ ions/cm²) kapton-H polyimide film in the visible region has been investigated at different temperatures ranging 400-2500 °C and at various electric fields ranging 40-600 kV/cm. A photoinduced exciton formation is the major source for providing charge carriers through thermolization and field-assisted dissociation processes. An attempt has been made to fit the field dependence of the steady state photocurrent to one of the several possible conduction mechanisms. In the high and low fluence (1.8 × 10¹³ and 1.8 × 10¹¹ ions/cm²) irradiated samples there exists a possibility of Poole-Frankel type of photoconduction mechanism, whereas at intermediate fluence (1.8 × 10¹² ions/cm²) a Schottky type photoconduction mechanism may be operative. The log I_ps versus 1/T plots consist of two straight lines with a knee point around 800-1000 °C. The activation energy estimated from the slope of these lines is field dependent varying from 0.40 to 0.73 eV and 0.18 to 0.23 eV above and below the knee point, respectively. This indicates the presence of more than one type of trapping levels in irradiated kapton-H polyimide.     

Study of surface activation of PET by low energy (keV) Ni and N ion implantation

Polyethyleneterephthalate (PET) has been modified by 100 keV Ni⁺ and N⁺ ions using metal ion from volatile compound (MIVOC) ion source to fluence ranging from 1 × 10¹⁴ to 1 × 10¹⁶ ions/cm². The increasing application of polymeric material in technological and scientific field has motivated the use of surface treatment to modify the physical and chemical properties of polymer surfaces. When a material is exposed to ionization radiation, it suffers damage leading to surface activation depending on the type. The surface morphology was observed by atomic force microscopy (AFM). That show the roughness increases with fluence in both the cases. The Ni particles as precipitation in PET were observed by cross-section transmission electron microscopy (XTEM). The optical band gap (E_g) deduced from absorption spectra; was calculated by Tau’c relation. Raman spectroscopy shows quantitatively the chemical nature at the damage caused by the Ni⁺ and N⁺ bombardment. The ration of I_D/I_G shows graphite-like structure is formed on the surface. A layer of hydrogenated amorphous carbon is formed on the surface, which has confirmed by XPS results also.     

Electron capture in Iq+ (q = 10–41) + He collisions at low energies

One electron capture processes in I^q+ (q = 10–41) +He collisions at low energies have been investigated. It is found that total cross sections for one-electron capture processes increase roughly linearly with increasing charge q of the incident ions and also increase with the square of the crossing radius R_c of the diabatic potential energy curves where the electron transfer takes place. These smooth increases are in contrast to those observed for ions with low charge q < 10.     

Properties of Na implanted targets

Ten types of ²³Na implanted targets have been fabricated for the purposes of investigating the effects of proton beam bombardment on the implanted sodium distribution. Targets were implanted at energies of E_Na = 10-30 keV using copper, tantalum, and nickel as host materials. Thin layers (100-200 Å) of chromium and gold were also evaporated over some of the targets to provide a protective layer for the implanted sodium. The ²³Na(p, γ) resonance at a lab proton energy of E_p = 309 keV was used to determine the implanted distribution. Successive resonance profile measurements are presented for each implanted target, and the concurrent loss of ²³Na resulting from beam bombardment is reported. The calculated temperature rise of the targets indicates that beam heating has a negligible effect on the implanted sodium distribution, and that the principal mechanism for ²³Na loss during beam bombardment is sputtering.     

Formation of silicon hydride using hyperthermal negative hydrogen ions (H) extracted from an argon-seeded hydrogen sheet plasma source

An E × B probe (a modified Wien filter) is constructed to function both as a mass spectrometer and ion implanter. The device, given the acronym EXBII selects negative hydrogen ions (H⁻) from a premixed 10% argon-seeded hydrogen sheet plasma. With a vacuum background of 1.0 × 10⁻⁶ Torr, H⁻ extraction ensues at a total gas feed of 1.8 mTorr, 0.5 A plasma discharge. The EXBII is positioned 3 cm distance from the sheet core as this is the region densely populated by cold electrons (T_e ∼ 2 eV, N_e ∼ 3.4 × 10¹¹ cm⁻³) best suited for H⁻ formation. The extracted H⁻ ions of flux density ∼0.26 A/m² are segregated, accelerated to hyperthermal range (<100 eV) and subsequently deposited into a palladium-coated 1.1 × 1.1 cm², n-type Si (1 0 0) substrate held at the rear end of the EXBII, placed in lieu of its Faraday cup. The palladium membrane plays the role of a catalyst initiating the reaction between Si atoms and H⁻ ions simultaneously capping the sample from oxidation and other undesirable adsorbents. AFM and FTIR characterization tests confirm the formation of SiH₂. Absorbance peaks between 900-970 cm⁻¹ (bending modes) and 2050-2260 cm⁻¹ (stretching modes) are observed in the FTIR spectra of the processed samples. It is found that varying hydrogen exposure time results in the shifting of wavenumbers which may be interpreted as changes in the frequencies of vibration for SiH₂. These are manifestations of chemical changes accompanying alterations in the force constant of the molecule. The sample with longer exposure time exhibits an additional peak at 2036 cm⁻¹ which are hydrides of nano-crystalline silicon.     

Nano-scale modification of 2D surface structures exposed to 6 keV carbon ions: Experiment and modeling

In this work, a Si pitch grating with typical lateral dimensions of 200-250 nm was exposed to 6 keV C⁺ ions at normal incidence and at an angle of 42° both parallel and perpendicular to the grating structure. In contrast to volatile and recycling ions (like Ar⁺ or H⁺), non-recycling ions are able to modify the surface not only due to sputtering, but also due to implantation of incident ions and the re-deposition of projectile atoms following sputtering or reflection. The target-projectile combination used in this work is an example of such a system forming a mixed Si-C surface. The interaction between the ion beam and the surface has been studied both experimentally and numerically with the focus on validation of the numerical model of the newly developed SDTrimSP-2D code. SDTrimSP-2D is capable of following the evolution of the Si-C system including ion-surface interactions with 2D micro- and nano-structured surfaces. The SDTrimSP-2D code takes the interdependency of surface morphology, sputtering and implantation into account. The simulated surface morphology has been compared to cross-sections of bombarded Si pitch grating obtained by SEM, revealing good agreement between experiment and simulation. The calculations also provide improved insight into the mechanisms of surface modification by sputtering, implantation and material transport by redeposition.